An appositive is a grammatical construction in which two elements, normally noun phrases, are placed side by side so one element identifies the other in a different way. An appositive is a noun or pronoun, often with modifiers, set beside another noun or pronoun to explain or identify it. Here are some key points about appositives:

• An appositive is a noun phrase that follows another noun phrase and provides additional information about it.
• An appositive may provide essential identifying information about the antecedent, in which case it’s called a restrictive appositive and is not separated from the surrounding text by any punctuation.
• The other kind of appositive is a nonrestrictive appositive, which provides extra information that is not needed to identify the antecedent and could be removed without changing the basic meaning of the sentence. It’s separated from the surrounding text with commas or other punctuation.
• Appositives are used to reduce wordiness, add detail, and add syntactic variety to a sentence.
• Appositives can come after a noun it renames, or they can stand as the introductory phrase of a sentence before the noun.
• Appositives are often one or two words, often including a name, but they can also be three or four words, often including some detail, or four or more words, often including extensive detail.

Overall, an appositive is a noun or pronoun that provides additional information about another noun or pronoun, and it can be either restrictive or nonrestrictive.